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4Stroke "Making Oil" by Willy B. Wright

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  • 4Stroke "Making Oil" by Willy B. Wright

    I get a few customers each year with high oil levels in their 4-stroke outboards and it's really tough to explain why sometimes. So I prepared this to present to customers with the problem and I thought it would make a good FAQ.

    Q: Why does my 4-stroke outboard oil level keep rising? I never add any. Shouldn't it be going down?
    A: This is referred to as "Making oil". Here it is in a nutshell. Water vapor enters the crankcase of all motors from the atmosphere, and as a by-product of combustion. In other motors, the oil gets rather hot and any water vapor that may condense will steam-off and exit the crankcase breather. 4-stroke outboard motor oil doesn't get nearly as hot, so the water just keeps on collecting.

    Q: Is it always water?
    A: No. Tiny amounts of raw fuel also leak into the crankcase on the compression stroke, potentially diluting the oil and raising the level on a cool-running motor. Or there could be a fuel system leak. Typically the odor of the latter situation is fairly obvious, but not necessarily. Lack of a gassy smell shouldn't preclude checking the possibility of fuel system leaks. Over choking and frequent flooding will also cause fuel to get into the oil.

    Q: Where does the water come from?
    A: Three sources. Asmosphere, combustion and fuel. There's always moisture in the air, especially near bodies of water. Some condenses out naturally and collects in the crankcase of a sitting motor. Some comes in right along with the air as it runs. The body of water you're operating on produces a lot of atmospheric water vapor, especially a few feet above the water line ... where the powerhead is. H20 is one of the compounds that forms when gasoline is burned, along with CO, CO2, NOX, etc. Some leaks into the crankcase right along with the other contaminants and mixes into the oil. Alcohol combustion makes mostly CO2 and H2O, so alcohol-blended fuels tend to produce more water vapor than straight gasoline.

    Q: Wouldn't my oil turn milky if water was in there?
    A: Eventually it will. But oil has some capacity to retain water and it eventually reaches a threshold where it starts to cloud.

    Q: Why doesn't the oil get hot enough to steam it off?
    A: It can. If the motor is running hard enough to plane the boat, water that otherwise cools the oil isn't splashing on the sump that holds the oil. But boats that are run at non-planing speeds may not heat the oil up very much at all. The water the boat is running in cools the oil which is in the sump beneath the powerhead. That water can keep the oil pretty cool just by splashing against the outside of the aluminum sump.Motors that are mostly operated at sub-planing speeds seem to be especially susceptible to making oil. Many times the complaint is that a kicker used for trolling has the problem, but the main motor doesn't. That's because the main motor is used to get to the fishing spot fast to troll slow with the kicker. One is hot, the other is not.

    Q: Why is it a problem with my motor and not my neighbor's?
    A: Two things really aggrivate the problem. Cool running temperature of the oil, and the abundance of moisture available that inevitably collects. The environment the boat is kept in, water temperature, the fuel you use, and the way you operate the motor have huge effects on whether or not this is going to be a problem.

    Q: Wouldn't a hotter thermostat solve the problem?
    A: Doesn't help much, but it helps. Problem is that you're regulating coolant temperature and not oil temperature. The oil will get a little warmer as a result, but the powerhead will be running hotter and that's probably not good. Then consider that if the sump is getting splashed by water, that pretty much cancels it out.

    Q: How can I be sure of what is making the oil?
    A: You can have the oil analyzed. There are many labs that test automotive oil. Outboard readings may not be consistent with what would be expected from an automobile engine, so interpreting the data could be problematic. But it sure will show what is in the oil. A quick search yielded a lot of labs offering this service through the mail for a wide range of prices. Shop around, but it could be money well invested. One site with abundant information is http://www.bobistheoilguy.com/ They also have a Forum if you have any questions.

    Q: So what can I do about it?
    A: Right now, the approach is to minimize the amount of contamination allowed to enter. That means keeping the motor as unexposed to atmospheric moisture as possible. Practical in some situations, impossible in others. The manufacturers are also stressing proper and agressive break-in procedures to mate surfaces better and keep leakage into the crankcase (blow-by) to a minimum. Avoid alcohol-blended fuels if you can. Finally, allow the motor to get hot enough to steam-off what water will inevitably collect in there. That'll probably mean running it hard for awhile. (If your lake has a speed limit ... Ouch!)If all of that fails, have the motor checked for gasket leakage. It could be coming from the cooling system. More frequent oil & filter changes may be necessary. But try running the motor hard for several minutes every time you go out. That's the easiest, cheapest, and most enjoyable thing to try.

    Q: So you mean I HAVE to open it up and go fast once in awhile?
    A: Dang shame, ain't it?

    Thanks to Bill, Bo, DJ, Gary, Jim, LubeDude, Ray, Robby, Rockhopper, TheOilDoc, and of course JB for their encouragement and invaluable assistance in preparing this FAQ
    Last edited by JB; October 3rd, 2007, 01:47 PM. Reason: reformatted by JustMrWill. thanks

  • #2
    Re: 4Stroke "Making Oil" by Willy B. Wright


    "Life should NOT be a journey to the grave with the intention of arriving safely in an attractive and well preserved body, but rather to skid in sideways, chocolate in one hand, martini in the other, body thoroughly used up, totally worn out and screaming "WOO HOO what a ride!"


    • #3
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      Re: 4Stroke "Making Oil" by Willy B. Wright

      I got this PM from Woodnaut on his experience with a correct prop correcting a "making oil" problem.
      I would like to submit a comment for your consideration. I was looking at the FAQ section, and particularly at the post on "Making Oil". This is an expert, well organized and lucid explanation of the causes of a four stroke making oil. I happen to have a 2006 Yamaha F90 and had this problem with it. As it turns out, it appears that a significant cause of my engine's problem was that I had it significantly over-propped which lowered by top end RPM to below 5,000. I went back to my old prop, WOT RPM went back up to nearly 6K and the problem instantly went away. The post alludes to this when it talks about combustion, but in my case over-propping caused such a dramatic effect that I wanted to mention it to you. Many Thanks and Best Regards - Woodnaut